Chemical slurry mechanical polishing (CMP) techniques are commonly used to provide planar and smooth surfaces on optical materials (such as for example, substrates for lithography photomasks) and other objects/materials. Fused silica and doped fused silica (such as for example titania doped fused silica) materials are commonly employed as substrates for lithography photomasks. For critical applications, photomasks used for lithography must have a high degree of surface smoothness (typically average roughness (Ra)<1.0 angstrom), with an accompanying high degree of surface planarity. CMP in combination with other secondary polishing techniques (such as laser polishing, dressed photon nano-polishing, magneto-rheological fluid polishing, or GCIB polishing, as examples) are capable of forming a smooth, planar surface on materials such as semiconductor materials, silica materials, etc. It is known that due to mechanical stresses occurring during the CMP process, shallow sub-surface defects can be created in the substrate material. Furthermore, small particles may become embedded in the substrate surface during CMP processing, though they may be flattened and planar with respect to the substrate surface. While the CMP and any subsequent secondary polishing leaves the surface smooth and planar, this sub-surface damage and/or embedded particles may have detrimental effects that limit the usability of the finished substrate. After the surface smoothing/planarizing steps (CMP or CMP plus secondary polishing) have been completed, the surface is cleaned with an intense agitation wet cleaning process (for example, ultrasonic solvent cavitation) to remove residual polishing slurry or other contaminating particles on the surface that could otherwise interfere with successful photomask formation, before proceeding with subsequent processing steps including, for example, depositions on the substrate. It is believed that during such aggressive cleaning processes, defects in the sub-surface damage region can propagate to the surface in the form of ‘pop outs’ that are released from the surface resulting in pits, depressions, or other smoothness defects that compromise the usability of the substrate as a photomask. Also, during aggressive cleaning, surface-embedded particles (polishing slurry materials or other contaminating particles) may be released from the surface leaving small pits in their previous locations. Often these defects do not become visible until later in the process after considerable processing expense has been incurred. Numerous approaches have been employed to find polishing and cleaning methods that reduce sub-surface damage while maintaining the necessary smoothness required, but without complete success in eliminating resultant surface defects that appear at later steps in the photomask fabrication process.
It is therefore an object of this invention to provide methods for treating a surface of a polished photomask substrate such as a polished photomask substrate (or other objects) to reduce defects by removing sub-surface damage prior to aggressive cleaning.
It is another object of this invention to provide polished and cleaned substrates such as for example photomask substrates (or other objects) to result in improved smoothness and reduced surface detects.